We often do assembly on the models we paint.
For tabletop standard we remove mould lines and repair obvious gaps in the models. But if people are on a budget this is often the first thing we recommend you do yourself.
For higher quality paint jobs we often go over the model with a fine comb and remove any blemishes on the model before painting.
If you want a high quality paint job we sometimes request the model to be assembled by us. Because then we have control over subassembly. This is sometimes also the case with intricate models.